Method of making combustion flash bulb

ABSTRACT

The ignition mass of combustion flash bulbs without an ignition wire is subjected to a breakdown by applying a relatively high voltage at the instant when it is in a vacuum or in an inert gas during manufacture. The finished bulb may be ignited by applying a voltage of between 10 and 100 V.

United States Paten 191 De Graaf et al.

[ METHOD OF MAKING COMBUSTION FLASH BULB [75] Inventors: Wilhelmus Polycarpus De Graaf,

Emmasingel, Eindhoven; Jacob Dirk Baars, Terneuzen, both of Netherlands [73] Assignee: U.S. Philips Corporation, New

York, NY.

22 Filed; Feb. 13, 1973 21 Appl. No.: 332,119

[30] Foreign Application Priority Data Feb. 19, 1972 Netherlands .Q 7202213 52105.0 ..316/26,316/22,431/95 51 Int. Cl. H0lj 9/00, F2lk 5/02 [58] FieldofSearch ..3l6/17,l8, 19,20,21;

[111 3, 23,994 [451 July 1 ,1974

[56] References Cited UNITED STATES PATENTS 3,752,636 8/1973 Warninck 431/95 Primary Examiner--Roy Lake Assistant Examiner-James W. Davie Attorney, Agent, or Firm-Frank R. Trifari [57] ABSTRACT between 10 and 100 V.

9 Claims, 5 Drawing Figures III The invention relates to a method of manufacturing a combustion flash bulb without an ignition wire, in which an ignition device is mounted within an envelope while forming a bulb base, said ignition device consisting of two current conductors sealed in the bulb base and terminating outside the envelope, the parts located within the envelope terminating in a mass consisting of a mixture of a metal powder, a binder and, optionally, an oxidant.

Combustion flash bulbs of this type are known as such and are described in literature. Generally these bulbs contain a filling of shredded metal which an ignition burns actinically in a gas maintaining the combustion, such as oxygen. However, it is alternatively possible to accommodate the actinically burning metal in the mass to be ignited by direct current passage.

Combustion flash bulbs of this type may in principle be manufactured in the same manner as combustion flash bulbs of the conventional type including an ignition wire connecting the ends of the current conductors located within the bulb. In the latterv bulbs the ignition mass is generally located at the ends of the current conductors. This is a result of the conventional manner of providing the ignition mass. To this end the ends of the current conductors which are connected by the ignition wire are immersed in-a suspension of the ignition mass. The suspension then creeps over the ends of the current conductor, but the surface tension generally prevents the suspension from coating the extremely thin ignition wire (diameter generally between and pm). It is evident that under these circumstances the mass is ignited when the ignition wire has only evolved sufficient heat to bring the mass at the ends of the current conductors at the ignition temperature. Thus, only part of the'energy dissipated in the bulb is necessary for the actual ignition. The ignition wire generally consists of a tungsten-rhenium alloy. To obtain a sufficient high resistance this wire is extremely thin and consequently thin and consequently weak. As a result of these causes the proportioning of this ignition system presents many difficulties. t

In a combination flash bulb in which the ignition mass is ignited by direct current passage through the ignition mass these difficulties do not occur. In addition these bulbs have the advantage that they can be electrically arranged in parallel and subsequently be ignited one by one by applying for a short period a voltage which quickly increases with time across the parallelarranged bulbs. Of the bulbs in the circuit which can still be flashed the bulb having the lowest ignition voltage will then'always ignite first while all electrical energy in this bulb is dissipated. For a satisfactory operation of such a circuit it is sufficient when for a mean ignition voltage of, for example, 1,000 V the ignition voltages mutually differ by several volts. This is ensured under normal manufacturing circumstances.

A drawback of this type of combustion flash bulb is, however, that the resistance of the ignition mass is 100 MO or more and that the ignition voltage is comparatively high, This means that strict requirements must be imposed on the insulation resistance of the circuit.

It is true that possible contact resistances at these high voltages can hardly be the cause of the fact that these bulbs do not ignite or do not completely ignite,

. 2 but yet the latter may take place in these bulbs due to other causes. For example, upon ignition the energy in the bulb may be depleted into the shredded metal filling if one'or more pieces thereof are too closely in the vicinity of the ignition mass or are in contact therewith.

An object of the invention is to provide a combustion flash bulb without an ignition wire in which these difficulties do not occur or occur to a'm'uch slighterextent.

According to the invention a bulb of this kind is characterized in that the mass in a vacuum or an inert gas atmosphere is subjected to an electrical breakdown by of a voltage applied across the terminals the resistance of the ignition mass decreases irreversibly. This is explained as follows. Considered electrically, it must be assumed that the ignition mass consists of a number of capacitors and resistors arranged in series and in paral- Iel. The capacitors are constituted by the metal particles and the interpositioned binder, possibly with the oxide films on the metal particles. When the voltage increases slowly the binder between the metal particles operating as a dielectric is probably broken down at a number of areas in the mass. These breakdowns are effected successively in case of a sufficiently slow increase of the voltage. In this case one or more conducting ducts are formed through the mass of a relatively low resistance. The heat evolved during the successive breakdowns has the opportunity to be conducted away so that the ignition mass does not have the opportunity to reach the combustion temperature. The result is that under thesecircumstances a larger energy is required for. combustion of the mass then when the voltage applied across the terminals is rapidly increased. In the latter case the binder is broken down simultaneously at a number of areas and locally a substantially adiabatic heating of the ignition mass may be effected so that the temperature in the masscan exceed the combustion temperature. According to the said German patent specification a too large part of the energy dissipated in the mass is used in an ignition mass having'a resistance of less than 1 MO so as to heat the current paths formed so that ignition as a result of the breakdown of the binder can no longer be effected. In this respect it is to be noted that the phenomena which are described in theGerman patent specification relate to combustion flash bulbs in which the distance between the terminals in the ignition mass is between 0.1 and 0.5- mm, preferably at 0.2 mm. In practice it has been found that flash bulbs for which the requirement applies that the distance e between the terminals is approximately 0.2 mm are difficult to manufacture in a reproducible manner. When according to the. present invention the electrical breakdown in the absence of a gas capable of reacting with the metal powder is preferably effected in 4 a vacuum, a current path of a-relatively low resistance is also formed. This is also the case if an oxidant is pres ent in the mass provided that the energy dissipated in the mass is chosen to be not too large, that is to say, preferably less than 30 pl.

specification No. I 1,255,487 which states that in case of a slow increase restriction that the voltage must be increased only slowly. The quantity of energy dissipated inthe mass is not liable to restrictions (in the absence of an oxidant) or is liable to restrictions to a lesser extent (in the presence of an oxidant). This means that a much lower final resistance of the ignition mass can be reached after electrical breakdown such as would be possible if the breakdown were effected in oxygen. 7

After the ignition mass has been subjected to an electrical breakdown in an oxygen atmosphere according to the invention it can already be brought to combustion by applying a direct voltage having a value between 10 and 100 V. This has the advantage that combustion flash bulbs according to the invention can be ignited, if desired, with the conventional dry-cell ignition devices.-

The invention particularly relates to combustion flash bulbs in which the distance between the ends of the current conductors is l mm i 0.3 mm and in which the resistance of the ignition mass not yet subjected to a breakdown measured between these ends at a voltage of 20 V is more than 10 Q.

Combustion flash bulbs according to the invention may be manufactured, for example, as follows.

In a glass tubular part a stem is provided, for example, consisting of two current conductors connected by means of a glass bead. The current conductors are sealed to one end of the tube while forming a bulb base. A drop of an ignition mass suspension is provided on the ends of the current conductors located within the tube, which drop connects these ends together. The liquid is evaporated and a tilling of metal chips is'introduced into the end, of the tube. Subsequently the tube is heated and reduced in area, the envelope space then formed is evacuated and possibly rinsed in an inert gas. At this stage, when there is no oxygen or another gas maintaining the combustion of the metal present in the lamp, the mass is subjected to an electrical breakdown.

A conducting duct through the ignition mass whose resistance is l 'to Q is produced between the ends of the current conductors. Subsequently the lamp is filled with oxygen and sealed.

According to a different method aglass tubular part is sealed at one end. The filling of shredded metal is inand the envelope space is shut off from the surroundings, for example, by pinching off the metal tube.

The stem may consist of two parallel wires which are mutually connected by means of a'glass bead or which are sealed in a cylindrical bodyprovided with a cavity for the ignition mass. The stem may alternatively consist of two'metal tubes one of which is located within the other. s

The ignition mass comprises a metal powder, for example, a powder of a metal such as zirconium, aluminium, magnesium and titanium, possibly an oxidant such as potassium perchlorate and a binder such as nitrocellulose and possibly a semiconducting material, activators etc. According to a preferred embodiment the ignition mass consists of astoichiometric mixture of zirco- 4 nium powderand potassium perchlorate (KCl0 and as a binder, for example, it is possible to use nitrocellulose. For example, 1 to 5 parts by weight of nitrocellulose in a dry state may be present per parts by weight of the stoichiometric mixture. The ignition mass is provided in the form of a suspension in an organic solvent on the ends of the current conductors.

A-suitable suspension may be obtained, for example,

bulb, which corresponds to approximately 0.8 mg zirconiumpowder and 0.3 mg potassium perchlorate.

The metal to be burned actinically may be, for example, zirconium or hafnium.

Before the bulb is filled with oxygen, the air is conducted away from it, for example, by rinsing with an inert gas and/or by evacuation. In this connection inert gases are understood to mean rare gases, nitrogen and gases which upon electrical breakdown of the mass do not react therewith. The gas maintaining the combustion consists of oxygen or a fluorine compound such as NF ,N F 0P or mixtures of such gases.

The ignition mass is preferably subjected to an electrical breakdown byconnecting the current conductors to a voltage source which provides a voltage pulse. increasing to a value in the order of 10 V within several microseconds.

. Before breakdown has taken place the resistance of the mass, measured at 20 V direct voltage, is more than 10 Q and after the electrical breakdown the resistance is only 10 to 10 Q.

A suitable voltage pulse may be obtained with the aid of apiezoelectric element.

The breakdown of the ignition mass may, under circumstances, alternatively be effected outside thebulb before the stem is mounted in the bulb. In that case it must be ensured that there is no combustion of the ignition mass during mounting. I

The invention will now be described in greater detail with reference to the accompanying drawings. The FIGS. 1 5 show in a cross-section successive stages in the manufacture of a combustion flash bulb according to a given embodiment of the method according to the invention. In a glass tube 1 (FIG. 1) having an outer diameter of 9 mm and a wall thickness of 1 mm a stem is provided consisting of two current conductors 2 and 3 and a glass body 5 having a cavity 4 which may beobtained, for example, by fusing a glass bead'pro'vided with the current conductors 2 and 3 with a glass tube (diameter 3 mm, length 5 mm). The body 5 is fused with the glass -1 while forming the bulb base 6. The ignition mass 7 (0.75 .4 litre) is provided in the cavity 4 (contents 1 1.1. litre) in which the current conductors 2 and 3 terminate by providing a drop of a suspension in the cavity 4. The suspension has, for example, the following composition l60 parts by weight of ziroconium powder 60 parts by weight of potassium perchlorate 2 parts by weight of nitrocellulose.

100 parts by weight of ethyl lactate (solvent).

After the greater part of the solvent has disappeared from the suspension by natural or forced evaporation, a filling of shredded metal 8 is introduced into the tube 1, for example, 20 mg of zirconium in the form of shredded foil having a width of approximately 40 ,um, a thickness of approximately 20 pm and a length of approximately cm. The glass tube 1 is reduced in area at 9 while it is being heated.

The tube 1 is connected to a vacuum pump and the I 600 V. The resistance is found to be less than 10 .Q

after breakdown.

After breakdown has taken place, the bulb is filled with oxygen up to a pressure'of 6,000 Torr and is sealed at 9 (FIG. 4).

FIG. 5 is a side view of the finished bulb. I

The bulb according to this embodiment may be ignited with the aid of a capacitor of approximately 100 ;LF charged to approximately V or more. It is alternatively possible to ignite the bulb with the previously mentioned voltage source (piezoelectric element) and the voltage then increases to approximately 100 V.

The combustion flash bulb has the advantage that it can be ignited with the same voltage source as a conventional combustion flash bulb having an ignition wire of tungstenrhenium. The relatively expensive wire is, however, superfluous in the combustion flash bulb according to the invention. An advantage is that no extra time-consuming stages during manufacturing are to be included. The breakdown ofthe ignition mass may take place during evacuation of the bulb which has for its purpose to conduct away the air therefrom and may be a step in the process of manufacturing the lamp in question.

A further advantage of the flash bulbs manufactured by the method according to the invention is that a possible contact between the metal shredded foil and the ignition mass in the bulb neither affects the ignition of the finished bulb nor the breakdown if this takes place in a vacuum during manufacture. This would be the case if in such bulbs the ignition mass were not subjected to an electrical breakdown in a vacuum in advance. Under the influence of external electrical fields voltage differences between the terminals in the ignition mass which lead to combustion cannot be produced in the flash bulbs according to the invention.

What is claimed is: g

1. In a method for the manufacture of a combustion flash bulb without an ignition wire, said bulb having an envelope, an ignition device and a finely divided combustible metal, said method including the steps of: forming a bulb base to said envelope while sealing two current conductors therein, said conductors having ends extending within said envelope to form a part of an ignition device and opposite ends terminating outside the envelope;

providing a quantity of an ignition mass consisting of a mixture of a metal powder and optionally, an oxidant and binder, on the ends of the conductors within the envelope;

and subjecting said ignition mass to an electrical breakdown in an atmosphere not supportive of combustion by applying a voltage across the current conductors.

2. The method of claim 1 wherein said atmosphere not supportive of combustion is a vacuum.

3. The method of claim 1 wherein said atmosphere not supportive of combustion is an inert gas.

4. The method of claim 1 also including the steps of:

filling said envelope with a finely divided combustible metal;

after subjecting said ignition mass to an electrical breakdown, filling said envelope with a gas which will maintain combustion and sealing said envelope.

5. The method of claim 4 wherein said combustible gas is oxygen.

6. A method as claimed in claim 1, wherein said mass consists of a stoichiometric mixture of zirconium and potassium perchlorate and a binder is used.

7. A method as claimed in claim 1, including the step of mounting an ignition device in the bulb so that the distance between the conductor ends located within the bulb is 1 mm i 0.3 mm and in which an ignition mass is used having an initial resistance between said ends greater than 10 0.

8. A method as claimed in claim 1, in which said step of subjecting the ignition mass to breakdown includes connecting the current conductors to a voltage source having an internal resistance which is of the same order of magnitude as the resistance of the ignition mass and which provides a voltage pulse rapidly increasing to a value of the order of 10 V within several microseconds.

9. A method as claimed in claim 1, in which said step of subjecting the ignition mass to breakdown includes subjecting said mass to an electrical breakdown with the aid of voltage pulse provided by a piezoelectric ele- 

1. In a method for the manufacture of a combustion flash bulb without an ignition wire, said bulb having an envelope, an ignition device and a finely divided combustible metal, said method including the steps of: forming a bulb base to said envelope while sealing two current conductors therein, said conductors having ends extending within said envelope to form a part of an ignition device and opposite ends terminating outside the envelope; providing a quantity of an ignition mass consisting of a mixture of a metal powder and optionally, an oxidant and binder, on the ends of the conductors within the envelope; and subjecting said ignition mass to an electrical breakdown in an atmosphere not supportive of combustion by applying a voltage across the current conductors.
 2. The method of claim 1 wherein said atmosphere not supportive of combustion is a vacuum.
 3. The method of claim 1 wherein said atmosphere not supportive of combustion is an inert gas.
 4. The method of claim 1 also including the steps of: filling said envelope with a finely divided combustible metal; after subjecting said ignition mass to an electrical breakdown, filling said envelope with a gas which will maintain combustion and sealing said envelope.
 5. The method of claim 4 wherein said combustible gas is oxygen.
 6. A method as claimed in claim 1, wherein said mass consists of a stoichiometric mixture of zirconium and potassium perchlorate and a binder is used.
 7. A method as claimed in claim 1, including the step of mounting an ignition device in the bulb so that the distance between the conductor ends located within the bulb is 1 mm + or - 0.3 mm and in which an ignition mass is used having an initial resistance between said ends greater than 108 Omega .
 8. A method as claimed in claim 1, in which said step of subjecting the ignition mass to breakdown includes connecting the current conductors to a voltage source having an internal resistance which is of the same order of magnitude as the resistance of the ignition mass and which provides a voltage pulse rapidly increasing to a value of the order of 103 V within several microseconds.
 9. A method as claimed in claim 1, in which said step of subjecting the ignition mass to breakdown includes subjecting said mass to an electrical breakdown with the aid of voltage pulse provided by a piezoelectric element. 